1. Field of the Invention
This invention relates to expanded polypropylene resin beads, to a method of producing a foam molding of expanded polypropylene resin beads and to a foam molding obtained by the method.
2. Description of the Prior Art
Expanded polypropylene resin beads are generally obtained by a dispersion method which includes dispersing particles (or pellets) of a polypropylene resin in water in an autoclave and impregnating the polypropylene resin particles with a blowing agent at a temperature higher than the softening point of the resin under pressurized conditions. The dispersion is then discharged from the autoclave to the atmosphere so that the resin particles are foamed and expanded. In an alternative method, a polypropylene resin is melted and kneaded in an extruder together with a blowing agent and the melt is extruded through a die into a lower pressure atmosphere so that the extrudate is foamed and expanded. The extrudate is cut into particles before or after the completion of the foaming and expansion thereof. A foam molding of expanded polypropylene resin beads may be produced by in-mold molding method in which the expanded beads are filled in a mold cavity defined by male and female molds or fed to a tunnel-like passage defined between a pair of running belts and serving as a mold cavity. A heating medium is then introduced into the mold cavity to expand (secondary expansion) and fuse-bond the expanded beads together. As the heating medium, steam is preferably used because of its easiness in handling and its high specific heat. The resulting foam molding is taken out of the mold cavity after it has been cooled down to such a temperature that it no longer expands when taken out of the mold cavity.
Such a foam molding is now utilized in various fields as an energy absorbing material, a heat insulating material, a cushioning material, a light weight container box because of its excellent physical properties. In these applications, the foam molding is also desired to have a good appearance, a smooth surface and a common degree of air or water permeation resistance. Excellent water permeation resistance is attained by fusion bonding of expanded beads without gaps or interstices. Excellency in water permeation resistance, however, causes a problem that a long time is required to cool down the foam molding in the mold cavity and, hence, the productivity is adversely affected. Namely, during the molding step, as the expanded beads are secondarily expanded and fuse-bonded to each other, the gaps therebetween are narrowed. In order to completely fuse-bond the expanded beads without gaps, it is necessary to use a large amount of steam. Therefore, after the molding step, it is necessary to cool the foam molding for a long period of time. This is disadvantageous from the standpoint of productivity.
To cope with this problem, Japanese Unexamined Patent Publication No. JP-A-2002-248645 proposes a method of producing a foam molding having a water permeation coefficient of 0 cm/sec while shortening a cooling time for the foam molding, which method includes preheating expanded thermoplastic resin small pieces having a through hole to increase the inside pressure thereof to 1.3 to 7 kgf/cm2, and then molding the pretreated small pieces in a mold by heating with steam.
The method of the Japanese publication has a problem because a pretreatment for increasing the inside pressure of the expanded small pieces must be carried out before the molding step at the sacrifice of productivity. The treatment is performed by placing the expanded small pieces in a pressurizing tank. After introducing a pressurized gas such as air or carbon dioxide into the tank, the expanded small pieces are allowed to stand under a pressurized state for several hours to several tens hours and, if necessary, with heating. Such a pretreatment with a pressurized gas is hereinafter referred to simply as “inside pressure increasing pretreatment”. Without such an inside pressure increasing pretreatment in the method of the Japanese publication, it is not possible to obtain a foam molding having the desired water permeation resistance, unless the expanded beads are filled in the mold cavity in a very excess amount under a compressed state, i.e. at a high compression ratio, as described in the Japanese publication. When a high compression ratio is adopted, however, the foam molding obtained has a considerably reduced expansion ratio (i.e. an increased density) and, further, the effect of shortening the cooling time is not fully satisfactory. Without such a pretreatment, it would be also possible to obtain a foam molding having the desired water permeation resistance, if the molding is performed using high temperature steam. In this case, however, the foam molding obtained causes considerable shrinkage and is not suited for practical use.